I am working to model mutations that underlie human cancer in the mouse, with the long term goal of harnessing these in vivo systems to identify biochemical pathways and target cell populations amenable to the development of more specific and less toxic therapies. In the laboratory of Kevin Shannon, M.D., I have chosen to investigate myeloid malignancies because many of the genetic events that contribute to leukemogenesis are known, hematopoiesis is a tractable experimental system, and current treatment strategies are largely unsatisfactory. Ras signaling is deregulated by various genetic mechanisms in myeloid malignancies including RAS point mutations, expression of aberrant tyrosine kinases, and inactivation of the NF1 tumor suppressor. Activating mutations in the HRAS, KRAS, and NRAS genes are the most common oncogenic lesions found in human cancer and are detected in approximately 30% of patients with myeloproliferative disorders (MPD), myelodysplastic syndrome, and acute myeloid leukemia. However, previous efforts to model RAS-induced myeloid leukemia in the mouse have yielded inconsistent results, with myeloid leukemia arising inefficiently or not at all. These and other observations suggested that oncogenic RAS is incapable of initiating leukemogenesis, and invariably is a secondary genetic lesion. To address this unresolved question, we exploited an allele engineered by our collaborators, who introduced a point mutation into the endogenous Kras locus and incorporated a loxP-stop-loxP (LSL) cassette that inhibits transcription. We used the Mx1-Cre strain to induce Cre-mediated excision of the LSL repressor element in hematopoietic cells. The presence of a single copy mutant allele within the endogenous Kras locus closely models the somatic mutations found in many human cancer cells. We found that inducing K-RasG12D protein expression rapidly causes a fatal MPD in Mx1-Cre LSL-KrasG12D mice with features of human myeloid malignancies. We will utilize this novel model to investigate the cellular and biochemical mechanisms of Ras-induced MPD by (1) characterizing the cellular phenotypes and signaling networks of hematopoietic cells expressing K-RasG12D, (2) identifying and purifying stem cells capable of initiating MPD, and determining which Ras effectors are expressed in these cells, and (3) specifically analyzing the role of signaling by the cytokine GM-CSF in establishment of MPD in vivo.